Fuel burners such as those found in water heaters, furnaces, boilers, etc. must have some sort of flame detector for safe operation. The danger resulting from fuel flowing into a combustion space without presence of a flame to burn the fuel is well known. These flame detectors have taken a variety of forms. For small burners such as water heaters and small furnaces, a thermocouple is perfectly adequate to detect the flame.
For larger burners though, the residual heat after flame is accidentally lost is sometimes sufficient to allow a thermocouple to continue to indicate flame. Accordingly, other mechanisms must be used to detect flame in these types of burners. A typical type of detector is the flame rod, which uses the difference in sizes of the metal burner itself and a small anode to function as a rectifier when AC power is applied across them.
Another type of flame detector relies on directly on the radiation provided by the flame. However, the mere presence of visible or IR radiation does not necessarily indicate an active flame. Walls of combustion chambers tend to radiate visible and IR energy for a period of time after flame is lost. It was found, however, that active flames have characteristic flicker frequencies in the IR, visible, and UV wavelengths. Typically, an active flame flickers in the 5 to 15 hz. range (as well as in higher frequencies) in all of these wavelength bands. Heated refractory walls or glowing particles have different flicker frequencies or none at all. So flicker in these wavelengths can be used to reliably indicate flame. One type of burner system flame detector using the flicker of the flame is described in U.S. Pat. No. 5,073,769.
We find that UV wavelengths are preferable for sensing of active flames for a number of reasons. Efficient combustion of hydrocarbon fuels produce flames that reliably emit UV radiation. When UV is detected, flame is always present, that is there are no false positives from combustion chamber walls or other sources. Presently, discharge tubes are used to detect UV radiation, but these require a high voltage power supply, and the tubes themselves have a relatively short operating life.
Solid-state UV detectors on the other hand are long lasting and operate on low voltage, but have a number of other undesirable characteristics. High gain or sensitive solid-state UV detectors lack temperature stability and do not have consistent electrical characteristics from one unit to the next. Low gain solid-state UV detectors are stable and have more consistent characteristics, but these provide very low signal output, typically in the tenths of a lamp. UV detectors that don""t have these disadvantages tend to be too expensive for flame detector applications. Accordingly, suitable solid-state flame detectors based on sensing UV radiation have not been available.
We have devised a circuit that can process the output of a low gain or low output UV sensor by using the UV radiation flicker in the sensor output, to thereby reliably detect when flame is present. The low gain UV sensor is of the type providing a raw UV signal varying with the level of UV energy impinging on the UV sensor. In a preferred embodiment, the UV sensor comprises a photodiode providing a sensing element signal comprising a varying low level current and includes a transimpedance amplifier receiving the sensing element signal and providing the raw UV signal as a varying voltage signal.
A capacitor receives the raw UV signal from the UV sensor and provides an AC UV signal following the changes in the raw UV signal but excluding at least a part of any DC component in the raw UV signal. A band pass amplifier receives the AC UV signal and provides a band pass amplifier signal encoding the frequencies within a preselected frequency range present in the AC UV signal. The band pass amplifier has a preferred frequency range of 5 to 15 Hz. We prefer a multistage band pass amplifier to provide better frequency rolloff at the boundaries of the preferred frequency range. A rectifier receives the band pass amplifier signal and provides a rectifier signal. Preferably, the rectifier is a full wave rectifier.
A final stage low pass filter receives the rectifier signal and provides a flame signal encoding the frequencies below a preselected frequency present in the rectifier signal. The level of the flame signal indicates the presence or absence of a flame. We prefer a low pass filter that blocks most frequencies above approximately 3-5 Hz.